add documentation for PyCharm Plugin

[koxudaxi]

Change Summary

The PR adds documentation for PyCharm Plugin.

Related issue number

#650

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.rst file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #750 into master will not change coverage.
The diff coverage is n/a.

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  Coverage     100%   100%           
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  Lines        2725   2723    -2     
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- Hits         2725   2723    -2

[dmontagu]

@koxudaxi Let me offer the following wordsmithing suggestions:

**plays nicely with your IDE/linter/brain**
    because *pydantic* data structures are just instances of classes you define;
    :ref:`auto-completion <pycharm_plugin>`, linting, :ref:`mypy <usage_mypy>` and your intuition
    should all work properly with your validated data.

(It seems awkward to me to call out PyCharm specifically this early in the docs, but I think it's okay to make a reference (I just put the link on "auto-completion"). I'd defer to @samuelcolvin though.)

.. _pycharm_plugin:

PyCharm Plugin
--------------

While pydantic will work well with any IDE out of the box, a PyCharm plugin
offering improved pydantic integration is available on the JetBrains Plugins Repository.
You can install the plugin for free from the plugin marketplace
(PyCharm's Preferences -> Plugin -> Marketplace).

The plugin currently supports the following features:

* For ``pydantic.BaseModel.__init__``:

     * Inspection
     * Autocompletion
     * Type-checking

* For fields of ``pydantic.BaseModel``:

    * Refactor-renaming fields updates ``__init__`` calls, and affects sub- and super-classes
    * Refactor-renaming ``__init__`` keyword arguments updates field names, and affects sub- and super-classes

More information can be found on the
`official plugin page <https://plugins.jetbrains.com/plugin/12861-pydantic>`_
and `Github repository <https://github.com/koxudaxi/pydantic-pycharm-plugin>`_.

Feel free to reject if you dislike/disagree with any of the changes.

[koxudaxi]

@dmontagu
Thank you reviewing my document.
I feel great about your sentence.
I have merged only the section of PyCharm Plugin.

@samuelcolvin
if you want to change the introduction section which is written dmontagu. (**plays nicely with you...) , then I merge one too.

[samuelcolvin]

otherwise looks great.

[koxudaxi]

@samuelcolvin
Thank you. I have committed!!

[samuelcolvin]

thank you very much.